An image capturing apparatus such as a digital camera that includes a single board color image sensor, for example, a CCD image sensor or CMOS image sensor and stores an optical image as digital image data is known. Although an image capturing apparatus like this is desirably capable of capturing high-quality images, noise (color noise) of a color component largely decreases the image quality in an environment in which no sufficient S/N ratio (signal/noise ratio) can be obtained, for example, in a dark place or at night. Another example of the factor that decreases the image quality is the superimposition of a noise component resulting from, for example, a dark current of an image sensor.
Demands, however, have particularly recently arisen for capturing low-noise, high-quality images even in a dark place or at night.
Under the technical circumstance as described above, various methods for suppressing color noise have been proposed. For example, Japanese Patent Laid-Open No. 2008-015741 (patent literature 1) has proposed a method of combining high-frequency components with low-frequency components after removing noise from the high-frequency components while saving edge components. Also, Japanese Patent Laid-Open No. 2007-272536 (patent literature 2) has proposed a method of generating a low-frequency image and high-frequency image different in resolution for each of a luminance component and color-difference component, performing different noise suppressing processes on these images, and then combining the images.
Each of the methods described in patent literatures 1 and 2 has an arrangement in which the luminance component and color component are divided into high- and low-frequency components, and combined after the noise suppressing processes are individually performed on them.
Furthermore, in the process of processing an image sensed by a single board color image sensor using color filters, a problem called a pseudo color arises in which an originally nonexistent color appears in a portion where an image of a subject having a high spatial frequency is captured. As a method of suppressing this pseudo color generated in a high-frequency region of a subject, therefore, color interpolation of a pixel of interest is conventionally performed using pixels around the pixel of interest.
Each of Japanese Patent Laid-Open No. 2002-300590 (patent literature 3) and Japanese Patent Laid-Open No. H08-023541 (patent literature 4) has proposed a method of performing color interpolation suppressing a pseudo color on an image obtained from an image sensor including color filters having a Bayer arrangement. The unit pattern of a primary color Bayer arrangement is
          ⁢                                   R                                      G            ⁢                                                  ⁢            1                                                            G            ⁢                                                  ⁢            2                                    B                    Of two green (G) filters included in this unit pattern, a G filter adjacent to a red (R) filter in the horizontal direction and to a blue (B) filter in the vertical direction is a G1 filter, and a G filter adjacent to the R filter in the vertical direction and to the B filter in the horizontal direction is a G2 filter.
When performing color interpolation on a pixel of interest, whether to use the value of a G1 pixel or G2 pixel in order to obtain a color-difference component is determined based on the relationship between the correlation degree of the value of the G1 pixel or G2 pixel vertically adjacent to the pixel of interest and the correlation degree of the value of the G1 pixel or G2 pixel horizontally adjacent to the pixel of interest.
In patent literature 4, a color-difference component R-G is obtained by performing weighted averaging on the value of (R-G1) and the value of (R-G2) in accordance with the difference between them, and a color-difference component B-G is obtained by performing weighted averaging on the value of (B-G1) and the value of (B-G2) in accordance with the difference between them.
Furthermore, Japanese Patent Laid-Open No. 2007-336384 (patent literature 5) has proposed a method of generating a luminance component by adaptively selecting a pixel for use in interpolation in accordance with the correlation of pixels positioned in the horizontal and vertical directions of a pixel of interest, thereby generating a natural image as a whole by suppressing moire.
Patent literatures 1 and 2 take the suppression of color noise into consideration, but do not take account of the suppression of a pseudo color. Also, patent literatures 3 to 5 take account of the suppression of a pseudo color, but do not take the suppression of color noise into account.
It is insufficient to suppress only one of color noise and a pseudo color, and an image in which both color noise and a pseudo color are suppressed is required. However, no conventional method for efficiently suppressing both color noise and a pseudo color has been proposed.